1. Field of the Invention
The present invention relates generally to multiple-input multiple-output (MIMO) communication systems, and more particularly to a MIMO communication system for selecting an optimal MIMO transmission and reception scheme and communicating using the selected scheme.
2. Discussion of the Related Art
Recent wireless communication technology improvements have been directed to various multimedia communications. Accordingly, a large amount of research is being conducted on high-speed data transmission technologies. The current third generation (3G) wireless communication systems are based on a code division multiple access (CDMA) transmission scheme. However, because the CDMA transmission scheme has limitations in processing large-capacity wireless data, a multicarrier based orthogonal frequency division multiplexing (OFDM) transmission scheme is being considered for next generation wireless communication systems.
A basic concept of the OFDM transmission scheme is to increase a data rate by converting a serial input data stream into N parallel data streams and transmitting the N parallel data streams through separated subcarriers.
Because the OFDM subcarriers are orthogonal, the spectra of subcarriers can overlap each other. A receiver can easily separate subcarriers using a simple signal processing technique. In this case, as compared with when data is sequentially transmitted using a single carrier, a transmission symbol interval is lengthened and the influence of a channel delay time or impulse noise is reduced. The OFDM transmission scheme can reduce interference between successive symbols, thereby offering the robustness to a multipath channel. Further, the OFDM transmission scheme can reduce the complexity of channel equalization, and can improve the spectral efficiency, as compared with a conventional frequency division scheme.
However, an increase in the transmission bandwidth alone is not sufficient to satisfy a target transmission rate for the next generation wireless communication systems. To increase a transmission rate for a given bandwidth, many methods have been presented. Currently, it is known that a method using multiple antennas is most effective. Although, multiple-input multiple-output (MIMO) multiplexing scheme for increasing a transmission rate using multiple antennas is difficult to be applied for wideband transmission. However, when OFDM characteristics are used, the MIMO multiplexing scheme can be applied for the wideband transmission. Accordingly, a large amount of research is being conducted on a method for combining MIMO multiplexing and OFDM, and new methods are being proposed.
In the wireless communication system, an increase in overall system throughput as well as an instantaneous transmission rate is important. A method that is most suitable to increase the overall system throughput is adaptive modulation and coding (AMC). AMC adapts to variations in a transmission environment. Also, in the next generation wireless communication systems, AMC is used as basis technology to improve the overall system throughput. AMC is suitable for an OFDM system because the OFDM system transmits different signals on a subcarrier-by-subcarrier basis and can be applied to subcarriers.
An increase in the transmission bandwidth alone is not sufficient to satisfy a target transmission rate for fourth generation (4G) wireless communication systems. To increase a transmission rate for a given bandwidth, many methods are present. As indicated above, it is known that a method using multiple antennas is most effective. Further, when OFDM characteristics are used, the MIMO scheme can be applied for the wideband transmission. Accordingly, a large amount of research is being conducted on a method for combining MIMO and OFDM, and new methods are being proposed.
The MIMO scheme can improve reception reliability for a given data rate or increase a data rate for given reliability. That is, diversity gain can be obtained by transmitting the same data through multiple paths in an environment in which a channel state is bad, and multiplexing gain can be obtained by transmitting individual data streams through different spatial channels in a parallel fashion in an environment in which a channel state is good.
Current research associated with MIMO is mostly focused on a transmission scheme for obtaining the maximum diversity gain or the maximum spatial multiplexing gain. Further, a technique for switching diversity mode and multiplexing mode according to channel variation has been proposed.
A link adaptation algorithm serving as one transmission mode switching technique is designed to maximize the spectral efficiency by switching between transmission diversity (TM) mode and spatial multiplexing (SM) mode based on signal to noise ratios (SNRs) and time/space indicators. However, the spatial selectivity of a channel is not considered in the link adaptation algorithm.
A multiplexing and diversity mode switching technique serving as another transmission mode switching technique based on the distance between signal points in the constellation has been proposed. In the constellation based transmission mode switching technique, a mode selection criterion uses the condition number of instantaneous channel realization as spatial information. However, this transmission mode switching technique may obtain significant diversity gain, but is based on a fixed transmission rate.